i3 config: Add gen_config.sh to generate machine specific configurations

Call ~/.i3/gen_config.sh <fragments...> from your ~/.xsession before
starting i3

The "base" config fragment is always included, you may pass more config
fragments as you need them. The fragments should go under ~/.i3/configs/

The reload & restart shortcuts will now call the gen_config.sh script
with whatever parameters it was originally given before reloading /
restarting i3. If you want to change the parameters to the script
without restarting your entire X session, run it manually with the new
parameters and manually call i3-msg reload.